1. Field of the Invention
The present invention relates generally to bone conduction devices, and more particularly, to a bone conduction device having a multilayer piezoelectric element.
2. Related Art
Hearing loss, which may be due to many different causes, is generally of two types, conductive and sensorineural. Sensorineural hearing loss is due to the absence or destruction of the hair cells in the cochlea that transduce sound signals into nerve impulses. Various prosthetic hearing implants have been developed to provide individuals who suffer from sensorineural hearing loss with the ability to perceive sound. One such prosthetic hearing implant is referred to as a cochlear implant. Cochlear implants use an electrode array implanted in the cochlea of a recipient to bypass the mechanisms of the ear. More specifically, an electrical stimulus is provided via the electrode array directly to the auditory nerve, thereby causing a hearing sensation.
Conductive hearing loss occurs when the normal mechanical pathways that provide sound to hair cells in the cochlea are impeded, for example, by damage to the ossicular chain or ear canal. However, individuals suffering from conductive hearing loss may retain some form of residual hearing because the hair cells in the cochlea may remain undamaged.
Still other individuals suffer from mixed hearing losses, that is, conductive hearing loss in conjunction with sensorineural hearing. Such individuals may have damage to the outer or middle ear, as well as to the inner ear (cochlea).
Individuals suffering from conductive hearing loss are typically not candidates for a cochlear implant due to the irreversible nature of the cochlear implant. Specifically, insertion of the electrode assembly into a recipient's cochlea exposes the recipient to potential destruction of the majority of hair cells within the cochlea. Typically, destruction of the cochlea hair cells results in the loss of residual hearing in the portion of the cochlea in which the electrode assembly is implanted.
Rather, individuals suffering from conductive hearing loss typically receive an acoustic hearing aid, referred to as a hearing aid herein. Hearing aids rely on principles of air conduction to transmit acoustic signals to the cochlea. In particular, a hearing aid typically uses an arrangement positioned in the recipient's ear canal or on the outer ear to amplify a sound received by the outer ear of the recipient. This amplified sound reaches the cochlea causing motion of the perilymph and stimulation of the auditory nerve.
Unfortunately, not all individuals who suffer from conductive hearing loss are able to derive suitable benefit from hearing aids. For example, some individuals are prone to chronic inflammation or infection of the ear canal thereby eliminating hearing aids as a potential solution. Other individuals have malformed or absent outer ear and/or ear canals resulting from a birth defect, or as a result of medical conditions such as Treacher Collins syndrome or Microtia. Furthermore, hearing aids are typically unsuitable for individuals who suffer from single-sided deafness (total hearing loss only in one ear). Hearing aids commonly referred to as “cross aids” have been developed for single sided deaf individuals. These devices receive the sound from the deaf side with one hearing aid and present this signal (either via a direct electrical connection or wirelessly) to a hearing aid which is worn on the opposite side. Unfortunately, this requires the recipient to wear two hearing aids. Additionally, in order to prevent acoustic feedback problems, hearing aids generally require that the ear canal be plugged, resulting in unnecessary pressure, discomfort, or other problems such as eczema.
As noted above, hearing aids rely primarily on the principles of air conduction. However, other types of devices commonly referred to as bone conducting hearing aids or bone conduction devices, function by converting a received sound into a mechanical force. This force is transferred through the bones of the skull to the cochlea and causes motion of the cochlea fluid. Hair cells inside the cochlea are responsive to this motion of the cochlea fluid and generate nerve impulses which result in the perception of the received sound. Bone conduction devices have been found suitable to treat a variety of types of hearing loss and may be suitable for individuals who cannot derive sufficient benefit from acoustic hearing aids, cochlear implants, etc, or for individuals who suffer from stuttering problems.